When ordering commercial aircraft, airline companies often specify a number of design options to the manufacturer, including passenger seating layouts. The manufacturing inventory costs associated with providing different seating arrangements and spacing between seats can be significant. These costs will become more important to the next generation of aircraft, which will offer new personal entertainment and service facilities to each passenger. Designers of such facilities are planning to install an entertainment system and passenger service system in the back of each aircraft seat for use by the passenger in the following seat. In order to supply power to these facilities, conventional wiring techniques will typically require that each different seating arrangement that an airline company specifies have a different length power lead harness. The cost and weight penalty associated with providing power for each seat using conventional wiring techniques is likely to be unacceptable to most passenger carriers. In addition to unacceptable costs in terms of weight, conventional wiring techniques inhibit the ability to rearrange classes of seating (by sliding seats to change seating pitch) in order to optimize the payload return on a flight (even within 30 minutes of departure). The ability to rearrange classes of seating on a short-term basis is a profitable feature that is desired by most airlines.
An alternative to wiring each seat to a power source utilizing a conventional power lead harness is disclosed in commonly assigned U.S. Pat. No. 4,428,078 (C. Kuo). This patent discloses what is referred to therein as a "wireless" system for supplying power to a plurality of multiple-turn pickup coils disposed in the base of seats throughout an aircraft cabin. Perhaps this technology could more accurately be described as a "connectorless" power supply system, because power is inductively coupled from a power supply loop that is disposed in the floor of an aircraft cabin to pickup coils attached to each seat group. The power is used to operate the passenger entertainment and service systems installed in the seats. This wireless system permits seats to be moved about in different arrangements as required by individual airline carriers, without concern for providing different length interconnecting wire harnesses.
In order for a connectorless power supply system to be practical in an aircraft environment, the system power source must meet certain criteria. The power source must have: (i) high efficiency (in order to maintain power dissipation low); (ii) very tight frequency regulation; (iii) reasonably well-regulated current amplitude over load and line variations; (iv) low weight; (v) low output and input wiring harmonic fluctuations; and (vi) high reliability. Furthermore, the power source must be producible for a reasonable cost and be fairly easy to manufacture.
High efficiency is mandated by the maximum allowable power dissipation in certain size enclosures dictated by present commercial aircraft specifications (ARINC 600). For an eight MCU enclosure, power dissipation must be less than 200 watts with forced air cooling. As a result, power dissipation is the requirement that must be met, not efficiency per se.
Fixed frequency is required because variable frequency would require autoresonating in the seat regulators of the aircraft, which adds cost and weight. More specifically, since a modern commercial aircraft has many seats, typically, many seat regulators are required. As a result, the weight and cost of seat regulators is a significant factor when it comes to choosing a power supply system. Varying frequency has the further disadvantage of causing interference with the RF transmission (and eventually the audio system) of an entertainment and service system. Because such interference will vary in an unpredictable fashion, its elimination is expensive and, therefore, undesirable.
The constant amplitude current requirement is also designed to make it easier for the seat regulators to maintain a constant DC output voltage. More specifically, the voltage induced in pickup coils is proportional to the current in a supply loop. If the supply loop current is maintained at a constant amplitude, the voltage induced at the pickup coils is relatively constant, whereby the job of the seat regulator to maintain a constant DC output voltage is eased. Contrariwise, if constant voltage is used as the power source requirement, the primary impedance associated with one seat pickup coil would consist of not only the power source output impedance and the supply loop inductance but also the impedances of all the other seat pickup units in the system. Because the impedances of the pickup units can vary widely, because required power can vary over a wide range as powered items (e.g., TVs) are turned on and off, the variation in pickup unit impedance would appear as a variation in source impedance. A varying source impedance increase would make it very difficult and, thus, more expensive for the seat regulators to maintain a constant output voltage.
A further factor that needs to be taken into consideration is the fact that contemporary aircraft power supplies generate 400 Hz power. An unacceptable number of supply loop and pickup coil turns are required to provide adequate power to seat regulators at 400 Hz. In order to reduce the number of turns in the supply loop and the pickup loop to an acceptable level, the 400 Hz power created by contemporary aircraft generators must be changed to a higher frequency. Obviously, too high a frequency cannot be chosen because of EMI considerations. More specifically, while higher frequencies increase the efficiency of a connectorless power supply system, in a commercial aircraft, the chosen fundamental frequency and, preferably, the third harmonic of the fundamental frequency must be below the frequency bands of the automatic direction finder and other navigation systems of the aircraft. In the past, 38 KHz was picked as a frequency low enough to avoid the EMI problem and to allow synchronization with the FM pilot of FM entertainment systems. The preferred embodiment of the present invention was designed based on using this frequency, i.e., 38 KHz, as the fundamental power switching frequency.